MICRO-OPTICAL LENSLETS BY PHOTO-EXPANSION IN CHALCOGENIDE CLASSES

A detailed study of the wavelength dependence of the formation of lens lets by photo-expansion in chalcogenide glasses is reported. Photo-exp ansion in chalcogenide glasses offers a one-step fabrication process t o record surface structures such as gratings and microlenses. The proc ess is purely optical and requires no fixing and etching to get the de sired surface structure. Two competing effects, namely large volumes a ccessed by low energy photons and large magnitudes of photostructural changes due to high energy photons, provide the possibility of using d ifferent fabrication techniques to obtain lenslets by the photo-expans ion process. Strongly absorbed 514.5 nm light is used to record micro- optical lenslets in As2S3 glass and up to 10% relative volume changes are observed. The low power density requirements at this wavelength ar e used to demonstrate a parallel lithographic fabrication technique fo r recording lenslets with high repeatability and throughput, and with excellent control over curvature and dimensions. The magnitude of surf ace dilation is maximized at the photon energy that combines the advan tage of short wavelength exposures in producing large percentage volum e changes, with the advantage of low energy photons in irradiating lar ge volumes of the glass matrix. A variety of wavelengths within the Ur bach tail range of the optical absorption edge are employed to find th e optimal wavelength for large absolute volume changes in As2S3 glass. Lenslets as high as 8 mu m are fabricated with focused light exposure s from a dye laser operating at 584.5 nm. Lenslets are structurally ch aracterized with alpha-step scanners and an atomic force microscope (A FM) and are optically characterized by testing the collimation of 1550 -nm light emerging from a single-mode fiber with these lenslets.